The field of the invention is retinal neuronal disorders.
Insulin-like growth factors (IGFs) have been identified in various animal species as polypeptides that act to stimulate growth of cells in a variety of tissues (Baxter et al., 1988, Comp. Biochem. Physiol. 91B:229-235; Daughaday et al., 1989, Endocrine Rev. 10: 68-91), particularly during development (D'Ercole, 1987, J. Devel. Physiol. 9:481-495). The IGFS, each of which has a molecular weight of about 7,500 daltons, are chemically related to human proinsulin: i.e. they possess A and B domains that (1) are highly homologous to the corresponding domains of proinsulin, and (2) are connected by a smaller and unrelated C domain. A carboxyterminal extension, the D domain, is also present in IGFs but is not found in proinsulin. Functional homologies between the IGFs and insulin also exist. Like insulin, IGFs stimulate phosphorylation of specific tyrosine residues within the cytoplasmic domain of the receptors to which they bind.
Using peptide specific antibodies as probes, IGF-I and IGF-II (sometimes respectively termed "somatomedin C" and "somatomedin A" have been found in a variety of tissues, including the mammalian central nervous system (CNS); the presence of mRNAs encoding these polypeptides in the CNS suggests local synthesis in the CNS (Baskin et al., 1988, TINS 11:107-111). In addition, IGF-III [or "brain IGF", or IGF-I(4-70], a truncated form of IGF-I lacking the latter protein's three N-terminal amino acid residues, has been found in fetal and adult human brain (Sara et al., 1986, Proc. Natl. Acad. Sci. USA 83:4904-4907) as well as in colostrum (Francis et al., 1988, Biochem. J. 251:95-103).
IGF receptors have been isolated from peripheral tissues as well as from brain tissue (Waldbillig, R. J. et al., 1988, Exp. Eye Res. 47:587-607; Massague, J. and M. P. Czech. 1982, J. Biol. Chem. 257:5038-5045; Rechler, M. M. and S. P. Nissley, 1985, Ann. Rev. Physiol. 47:425-442). The receptors found in the cell membrane are either dimers, comprised of one alpha and one beta subunit, or heterotetramers, comprised of two alpha/beta subunit pairs. Although IGFs bind to the dimeric form of the receptor, functional activation occurs only upon binding to the heterotetrameric species (Tollefsen, S. E. et al., 1991, Biochemistry. 30:48-54). IGF receptors isolated from peripheral and brain tissue differ in the molecular weights of their alpha subunits (Waldbillig, R. J. et al., 1988, Exp. Eye Res. 47:587-607), and even within brain tissue, IGF receptors isolated from neuronal cells are different to those isolated from glial cells (Burgess, S. K. et al., 1987, J. Biol. Chem. 262:1618-1622). Whether these differences reflect altered functional or binding specificities is not known. Finally, European Patent Application No. 86850417.6 describes evidence for a another type of IGF receptor located in human fetal membranes.
IGF-I and IGF-II appear to exert a stimulatory effect on development or proliferation of a wide range of susceptible cell types (Daughaday et al., Supra). Treatment with IGFs, or with certain polypeptide fragments thereof, has been variously suggested as a bone repair and replacement therapy (European Patent Application No. 88303855.6), as a means to counteract certain harmful side effects of carcinostatic drugs (Japanese Patent Application No. 63196524), and as a way to increase lactation and meat production in cattle and other farm animals (Larsen et al., U.S. Pat. No. 4,783,524). The effects of IGF on cells obtained from various parts of the CNS, and from the peripheral nervous system has been studied (Aizenman et al., 1987, Brain Res. 406:32-42; Fellows et al., 1987, Soc. Neurosci. Abstr. 13:1615; Onifer et al., 1987, Soc. Neurosci. Abstr. 13:1615; European Patent Application No. 86850417.6; Bothwell 1982, J. Neurosci. Res. 8:225-231; Recio-Pinto et al., 1986, J. Neurosci. 6:1211-1219). In addition, the IGFs have been shown to affect the development of undifferentiated neuronal-like cells: When IGFs were added to the growth medium of human neuroblastoma tumor cells, these cells were observed to extend neurites and to undergo mitosis (Recio-Pinto and Ishii, 1988, J. Neurosci. Res. 19:312-320; Mattson et al., 1986, J. Cell Biol. 102:1949-1954).
Within nervous tissue, IGFs have been shown to induce glial cell enzyme activities (McMorris et al., 1985, J. Neurochem. 44:1242-1251), to induce differentiation and development of oligodendrocytes (McMorris and Dubois-Dalcq, 1988, Neurosci. Res. 21:199-209), and to support embryonic brain cell proliferation, development and neurite outgrowth (Neilsen, F. and S. Gammeltoft, 1990, FEBS Letters 262:142-144; Svrzic and Schubert, 1990, Biochem. Biophys. Res. Comm. 172:54-60; Torres-Alwman, et al., 1990, Neuroscience 35:601-608; Recio-Pinto et al., 1986, J. Neurosci. 6:1211-1219).
IGFs have been found in both the developing and adult eye in the aqueous (Tripathi et al., 1991, Dev. Drug Res. 22:1-23) and vitreous humor (Grant et al., 1991, Diabetes 35:416-420). Autoradiographic studies using iodinated peptides revealed IGF binding sites within the uveal tract, choroid, lens, sclera and retina (Bassas, et al., 1989, Endocrinology 125:1255-2320; Bassnett and Beebe, 1990, Invest. Ophthalmol. Vis. Sci. 31:1637-1643; Waldbillig, et al., 1990, Invest. Ophthalmol. Vis. Sci. 31:1015-1022). In the adult retina, IGF-I binding sites appear to be specifically localized to the nuclear layers, and the photoreceptor regions, including the rod outer segments (Ocrant, et al., 1989, Endocrinology 125:2407-2413; Waldbillig, et al., 1988, Exp. Eye. Res. 47:587-607; Zick et al., 1987, J. Biol. Chem. 262:10259-10264), whereas proteins immunologically related to IGF-II receptors have been demonstrated in the retinal pigment epithelium (Ocrant et al., 1989, Endocrinology 125:2407-2413). IGF-I and IGF-II MRNA levels are highest within the retina of the eye (Danias and Stylianopoulou 1990, Curr. Eye Res. 9:379-386). However, the function of IGFs in the eye is unknown and the IGF binding sites in the retina have not been fully characterized. Therefore, it is not yet known whether these sites actually function as IGF receptors, i.e. whether they mediate a biological response.
It has been speculated, based upon results establishing that IGF-I affects the permeability of membranes for potassium (Beebe et al., 1986, Prog. Dev. Biol. Part A: 365-369; Parmelee and Beebe, 1988, J. Cell Phys. 134; 491-496) and that outer and inner rod segments contain IGF binding sites (Waldbillig et al., 1988, Exp. Eye. Res. 47:587-607; Zick et al., 1987, J. Biol. Chem. 262:10259-10264), that IGF-I might be involved in light transduction.
With regard to diabetic retinopathy, where the major pathological finding in the eye is neovascularization, King et al. (1985, J. Clin. Invest. 75:1028-1036) state that "In the present study, we have characterized the receptors and the growth promoting effect of insulin-like growth factor (IGF-I) and multiplication-stimulating activity (MSA, and IGF-II) on endothelial cells and pericytes from calf retinal capillaries and on endothelial and smooth muscle cells from calf aorta.," and, "These data show that vascular cells have insulin and IGF receptors but have a differential response to these hormones. These differences in biological response between cells from retinal capillaries and large arteries could provide clues to understanding the pathogenesis of diabetic micro-and macroangiopathy". In addition, Grant et al. (1986, Diabetes 35:416-420) state that "The concentrations of IGF-I in the vitreous of most diabetic subjects with severe neovascularization are thus in the range known to stimulate cellular differentiation and growth in several systems. Whether they do so in the eye, and thus contribute to the development of retinopathy, remains to be determined".